Most people here say that the Pentium Ds throttle at 72C or so. However, in my old 820 system, it had to get up to 78C before it throttled at all. In my current system, I have run testing and have confirmed it to not be throttling even as high as 80C. My current system has a different motherboard than the one where it throttled at 78C though. This leads me to believe that it is possible that the motherboard itself has some influences on the throttling.
As far as I know, it is impossible to disable throttling.
For reference, I run my 805 at 3.52Ghz at 1.4volts with a modified Big Typhoon cooler. It reaches around 66C at full load and 41C idle.